Tie-back connection for subsea well

ABSTRACT

A connection structure is disclosed for connecting a riser to a subsea wellhead assembly. The connection structure allows components of the tie-back connector ( 16 ) to be assembled over the tie-back connector before the tie-back connector is secured to a long stress joint, to avoid the need to maneuver the components over the long stress joint. High strength materials are used in the construction of the stress joint to minimize the effects of stresses in the wellhead connector. The connection fixture provides electrical insulation ( 34 ) between the engagement points of the stress joint and the tie-back connector to minimize corrosive galvanic action. The connection structure provides an upwardly facing flange ( 18 ) that may be threaded or otherwise connected to the top of the tie-back connector and is adapted to mate with and secure to a downwardly facing flange ( 12   a ) carried at the bottom of the stress joint ( 12 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims benefit of U.S. ProvisionalApplication Ser. No. 60/409092 filed Sep. 9, 2002 and assigned to theAssignee of the present application.

FIELD OF THE INVENTION

The present invention relates to connectors for securing riser pipes tosubsurface wellheads. More specifically, the present invention relatesto an improved connection between a riser stress joint and a tie backconnector.

BACKGROUND OF THE INVENTION

An external string of pipe is frequently used to communicate fluids andwell products between a subsea wellhead and a drilling or productionstructure located at the water surface. This string of pipe, frequentlyreferred to as a riser, is subjected to strong lateral and longitudinalstresses caused by water currents and motion between the surfacestructure and the wellhead. Because the wellhead is stationary relativeto the riser, these stresses concentrate in the connection between theriser and the wellhead.

Substantial effort has been exerted in the design of the structure usedto connect the lower end of the riser string to the wellhead to minimizethe effects of the stresses acting through the riser. A stresscompensating tie-back connector may be used to tie back a subsea well toa floating surface structure such as a SPAR or tension leg platform. Inthese installations, the lower end of the riser connects to the top of astress joint that is designed to connect to the wellhead to absorb partof the stresses exerted by the riser. The riser, stress joint and tieback connector are all run as a unit that is lowered through the waterfrom the surface structure and secured to the subsea wellhead.Conventional prior art assemblies frequently construct the upper tieback connector body and the stress joint as a single piece component.

Where the upper tie back connector body and the stress joint areconfigured as a single piece component, difficulties can be encounteredin the assembly of hydraulic actuation components or other mechanismsforming part of the tie back connector. This problem may be encountered,for example, where hydraulic lock actuation mechanisms used for securingthe riser to the wellhead must be assembled over the top of the tie-backconnector. Moreover, optimum stress reaction characteristics of thestress joint may not be attainable when the upper tie back connectorbody and the stress joint are constructed as a single piece component.

Constructing the upper tie back connector body and the stress joint ofdifferent materials, which could improve the stress capacity of theconnection, introduces the problem of increased corrosion caused by thegalvanic action resulting from the joinder of dissimilar materials in asaltwater environment.

SUMMARY OF THE INVENTION

A connection structure is provided between a stress joint in a riser anda tie-back connector of a subsea wellhead. The connection structurejoins a stress joint of one material with a wellhead tie-back connectorof a different material. The connection structure allows components ofthe tie-back connector to be assembled over the tie-back connectorwithout first having to be maneuvered over a long stress joint.

High stress capacity materials are used in the construction of thestress joint to minimize the effects of riser generated stresses in thewellhead connector. The connection fixture provides electricalinsulation between the engagement points of the stress joint and thetie-back connector to minimize corrosive galvanic action.

In the preferred embodiment, the connection structure provides anupwardly facing flange extending from the top of the tie-back connectoradapted to mate with and secure to a downwardly facing flange at thebottom of the stress joint. The upwardly facing flange is preferablythreaded to the top of the tie-back connector.

In a modified form of the Invention, mechanically actuated radial dogsor C-ring locking members are used to secure the upwardly facing flangeto the top of the tie-back connector.

From the foregoing, it will be appreciated that a primary object of thepresent invention is to provide a connection structure to join a stressjoint to a tie-back connector of a subsea wellhead.

Another object of the present invention is to provide a connectionstructure to join a stress joint of one material to a tie-back connectorof a different material.

Yet another object of the present invention is to provide a connectionstructure that permits components of a tie-back connector to beassembled without the obstruction provided by a stress joint integrallyconnected to the tie-back connector.

It is also an object of the present invention to minimize corrosion inan assembly of dissimilar metals in a connection between a subsea welland a stress joint.

The foregoing features, advantages and objects of the present inventionwill be more fully understood and better appreciated by reference to thefollowing address vacation and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a connection structure of thepresent invention secured to the upper body (indicated in dotted line)of a tie-back connector.

FIG. 2 is an enlarged vertical sectional view of the connectionstructure of FIG. 1.

FIG. 3 is a vertical sectional view of a modified form of the connectionstructure of the present invention.

FIG. 4 is a vertical sectional view of yet another modified form of theconnection structure of the present invention.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 illustrates a preferred embodiment of the connection structure ofthe present invention, indicated generally at 10, for connecting astress joint 12 to a subsea wellhead assembly (not illustrated). Anupper body of a tie-back connector, indicated generally at 14, extendstoward the water surface from the subsea wellhead assembly. An exampleof the tie-back connector 14 is disclosed in U.S. patent applicationSer. No. 09/954,998, now published in Pub. No. U.S. 2002/0096878,incorporated herein by reference for all purposes.

The tie-back connector 14 includes an elongate tubular tie-back body 16and a tie-back flange 18 mechanically secured to the tie-back body 16.The tie-back flange 18 has a mating face 22 facing upwardly from thetie-back connector 14 for connecting to a flange 12 a at the base of thestress joint 12.

The mechanical connection securing the connection structure 10 and thetie-back body 16 may comprise a threaded connection 28 between thetie-back flange 18 and the tie-back body. The threaded connection 28 maybe a square thread or a buttress thread, and is preferably comprised ofsubstantially the same material as the tie-back body 16. The stressjoint flange 12 a has a mating face 26 facing downwardly from the stressjoint 12 for mating with the mating face 22 of the tie-back flange 18.Threaded fasteners 31, such as threaded studs 30 and nuts 32, may securethe stress joint flange 12 a to the tie-back flange 18. The studs arethreadably received within tapped bores formed in the tie-back flange18. A gasket 36 provides a seal between the stress joint flange 12 a andthe tie-back flange 18.

The stress joint 12 including stress joint flange 12 a is preferablyconstructed of a material, such as a titanium alloy, that exhibits highresistance to structural failure caused by fatigue damage. The top ofthe stress joint portion shown in the figures may be welded to anextended portion of stress joint. The tie-back body 16, including theflange 18, may be constructed of a much less expensive low alloy steel.The physical connection of the steel and titanium alloys in the presenceof saltwater can produce damaging electrical currents that are conduciveto rapid corrosion and destruction of the connection. The effects ofthis phenomenon are minimized by providing electrical insulators toseparate the engagement points of the two metals. To this end, anelectrically non-conductive bent insulating ring 34 is disposed betweenthe tie-back body 16 and the stress joint flange 12 a. The insulationring 34 may comprise an upper sealing surface for sealing with thegasket 36 and for insulating between the gasket 36 and the riser flange12 a. The insulation ring may further serve as a spacer to space apartportions of mating faces 22 and 26 to prevent electrical contact betweenthem (as shown, outer portions of mating faces 22 and 26 are angled tocreate a gap between them. In other embodiments, the insulation ring 34may extend further between mating faces 22 and 26, to insulate greaterarea. Additionally, insulation washers 38 are positioned around thethreaded studs 30, between the nuts 32 and the stress joint flange 12 a,to complete the electrical isolation between the titanium stress joint12 and the low alloy steel tie-back body 16. The insulators 34 and 38may be constructed from any suitable material providing the desiredisolation to electrical current flow between the dissimilar metals whilesimultaneously providing the necessary structural strength to withstandthe compression imposed during the bolting together of the flangedcomponents. Examples of such insulation material include ceramic orceramic-coated steel. Additionally, an OD of the threaded studs 30 ispreferably smaller than the mating hole in the flange 12 a through whichthey pass, to create a gap around threaded studs 30 and preventelectrical contact between threaded studs 30 and flange 12 a.

FIG. 3 illustrates a modified embodiment of the present inventionindicated generally at 110. As with the embodiment of FIGS. 1 and 2, theconnection structure 110 includes a tie-back connector indicatedgenerally at 114 having an elongate tubular tie-back body 116 and atie-back flange 118 mechanically connected to the tie-back body.Components of the embodiment of FIG. 3 that correspond with those of theembodiment of FIGS. 1 and 2, and function in the same or similar manner,have been accorded reference characters that are greater by 100 than thecorresponding reference characters used in FIGS. 1 and 2. The tie-backbody 116 terminates in a threaded male connection 128 that is threadablyengaged with the tie-back flange 118 to form an upwardly facing flangeface at the upper body of the tie-back connector 116. A riser flange 112a is formed at the base of a riser joint 112.

The stress joint flange 112 has a mating face 126 facing downwardly fromthe riser 112 to a sandwich flange 134 held between the stress jointflange 112 a and the tie-back flange 118. Threaded fasteners 131including studs 130 extend through flange openings in the flange 112 aand sandwich flange 134 into threaded receptacles in the tie-back flange118. Nuts 132 are tightened onto the studs 130 to secure the componentstogether. Compression seals 136 and 136 a prevent leakage in theconnection structure 110.

The sandwich flange 134 is constructed of an electrical insulatingmaterial such as described above to prevent the generation of galvaniccurrents between the stress joint 112 and the tie-back body 116. Theinsulating washers 138 prevent current flow through the studs 130.

FIG. 4 illustrates another embodiment of the connection structure of thepresent invention indicated generally at 210. As with the embodiment ofFIGS. 1-3, the connection structure 210 includes an elongate tubulartie-back body 216 and a tie-back flange 218 mechanically connected tothe tie-back body 216. Components of the embodiment of FIG. 4 thatcorrespond with those of the embodiment of FIGS. 1-3, and function inthe same or similar manner, have been accorded reference characters thatare greater by 100 than the corresponding reference characters used inFIG. 3. In contrast to the threaded connections 28 and 128 used toconnect the tie-back bodies 16, 116 with the tie-back flanges 18, 118,respectively, in FIGS. 1, 2, and 3, the connection structure 210 employsa radially movable latch member 228 for connecting the tie-back body 216and the tie-back flange 218. The latch member 228 is constructed withupper and lower sets of circumferentially extending teeth 230 and 232,adapted to respectively engage grooves 231 and 233 about adjacent endsof the tie-back body 216 and the tie-back flange 218 when the latchmember 228 is moved radially inward. The latch member 228 may comprise aplurality of mechanically actuated dogs or a one-piece C-ring lockingmember.

The latch member 228 may be retained in the radially inward positionillustrated in FIG. 4 by a tubular retaining member 250. The retainingmember is secured with retaining member 251 including threaded studs 252and nuts 254. The threaded studs 252 are received in tapped receptaclesformed in the flange connector 218.

Radial movement of the latch member 228 into the radially inwardposition may be caused by axial movement of the retaining member 250relative to the latch member 228, such as occurs when the bolts 254 aretightened on the studs 252. Mating cam surfaces 260, 261 on the latchmember 228 and the retaining member 250 slidably engage during the axialmovement of the retaining member 250 to cause the radial movement of thelatch member 228.

The tie-back flange 218 has an upwardly facing mating face 222 forconnecting to a stress joint 212. A stress joint flange 212 a at thebase the of the joint 212 has a mating face 226 facing downwardly formating with the mating face 222 of the tie-back flange 218. Studs 230and nuts 232 secure the joint 212 to the tie-back flange 218. Aninsulation ring 234 between the opposing faces and 222 and 226 provideselectrical insulation between the engagement points of the stress joint212 and the tie-back flange 218. Insulating washers 238 complete theelectrical insulation of the dissimilar metals of the connectionstructure from each other.

Although specific embodiments of the invention have been describedherein in some detail, it is to be understood that this has been donesolely for the purposes of describing the various aspects of theinvention, and is not intended to limit the scope of the invention asdefined in the claims which follow. Those skilled in the art willunderstand that the embodiments shown and described are exemplary andvarious other substitutions, alternations, and modifications, includingbut not limited to those design alternative specifically discussedherein, may be made in the practice of the invention without departingfrom the spirit and scope of the invention.

1. An apparatus for connecting a riser to a subsea wellhead assembly,comprising: a tie-back connector for connection to the wellheadassembly, the connector including an elongate tubular tie-back body anda connector assembly radially outward of the tubular tie-back body; anda tie-back flange assembly having first and second flanges, with saidfirst body flange having a connection for separable connection to anaxial end of the tie-back body and said second riser flange forconnecting to the riser, said tie-back body and said riser each formedfrom a metallic material, said riser and said riser flange beingremovable from said body flange, and said body flange being removablefrom said tie-back body for insertion or removal of said connectorassembly on said tie-back body.
 2. An apparatus as defined in claim 1,further comprising: the riser flange mechanically connected to theriser, the riser flange having a mating face for engagement with thetie-back body flange; and a securing structure for removably securingthe riser flange to the tie-back body flange at their mating faces. 3.An apparatus as defined in claim 2, wherein the tie-back body and theriser flange are of dissimilar materials.
 4. An apparatus as defined inclaim 3, further comprising: an insulator for electrically insulatingbetween the tie-back body and the riser flange.
 5. An apparatus asdefined in claim 4, wherein the insulator comprises: an insulation ringbetween the tie-back body and the riser flange.
 6. An apparatus asdefined in claim 4, wherein the insulator comprises: one or moreinsulation washers positioned between a corresponding one or morethreaded members joining the riser flange with the tie-back body flange.7. An apparatus as defined in claim 1, further comprising: a threadedconnection between the tie-back body flange and the tie-back body. 8.(canceled)
 9. An apparatus as defined in claim 81, further comprising: asandwich flange for positioning between the mating faces of the tie-backbody flange and the riser flange.
 10. An apparatus as defined in claim9, wherein the sandwich flange comprises: an insulating material forelectrically insulating between the riser flange and the tie-back bodyflange.
 11. An apparatus as defined in claim 1, further comprising: aradially movable latch member for connecting the tubular tie-back bodyand the tie-back body flange, the latch member comprising upper andlower teeth adapted to engage grooves about adjacent ends of the tubulartie-back body and the tie-back body flange when the latch member ismoved radially inwardly.
 12. An apparatus as defined in claim 11,wherein the radially movable latch member comprises: mechanicallyactuated dogs or a C-ring locking member.
 13. (canceled)
 14. (canceled)15. An apparatus as defined in claim 1, further comprising: a sealmember for sealing between the riser flange and the tubular tie-backbody.
 16. An apparatus as defined in claim 1, wherein the tie-back bodyis a low alloy steel and the riser flange is a titanium alloy.
 17. AnApparatus for connecting a riser to a subsea wellhead assembly,comprising: a tie-back connector for connection to the wellheadassembly, the tie-back connector including an elongate tubular tie-backbody and a connector assembly radially outward of the tubular body; atie-back body flange mechanically connected to the tie-back body, thetie-back body flange having a mating face facing upwardly from thetie-back connector for connecting to the riser, the tie-back body flangecomprising different materials than materials comprising the riser; ariser flange connected to one axial end of the riser, the riser flangehaving a mating face facing away from the riser, for mating with themating face of the tie-back body flange, said tie-back body and saidriser each formed from a metallic material, said riser and said riserflange being removable from said body flange, and said body flange beingremovable from said tubular body for insertion or removal of saidconnector assembly on said tubular body; a threaded connection betweenthe tie-back body flange and the tie-back body; and an electricalinsulation material separating the different materials of the tie-backconnector and the riser flange.
 18. Apparatus for connecting a riser toa subsea wellhead assembly, comprising: a tie-back connector forconnection to the wellhead assembly, the tie-back connector including anelongate tubular tie-back body and a connector assembly radially outwardof the tubular body; a tie-back flange assembly having first and secondflanges, with said first body flange having a connection for separableconnection to an axial end of the tie-back body and said second riserflange having a mating face for connecting to a riser; said tie-backbody and said riser each formed from a metallic material; a riser flangemechanically connected to the riser, the riser flange having a matingface facing downwardly from the riser, said riser and said riser flangebeing removable from said body flange, and said body flange beingremovable from said tie-back body for insertion or removal of saidconnector assembly on said tie-back body; a threaded connection betweenthe tie-back body flange and the tie-back body; a sandwich flange forpositioning between the mating faces of the tie-back body flange and theriser flange, the sandwich flange comprising an insulating material forelectrically insulating between the riser flange and one or more of thetubular tie-back body and the tie-back body flange; one or more threadedmembers passing through and joining the sandwich flange, the riserflange, and the tie-back body flange; and one or more insulation washerspositioned between a corresponding one or more threaded members joiningthe riser flange with the tie-back body flange.
 19. Apparatus forconnecting a riser to a subsea wellhead assembly, comprising: a tie-backconnector for connection to the wellhead assembly, the tie-backconnector including an elongate tubular tie-back body and a connectorassembly radially outward of the tubular body; a tie-back body flangemechanically connected to the tie-back body, the tie-back flange havinga mating face facing upwardly from the tie-back connector for connectingto the riser, said tie-back body and said riser each formed from ametallic material; a riser flange mechanically connected to the riser,the riser flange having a mating face facing downwardly from the riserfor mating with the mating face of the tie-back flange, said riser andsaid riser flange being removable from said body flange, and said bodyflange being removable from said tie-back body for insertion or removalof said connector assembly on said tie-back body; a radially movablelatch member for connecting the tubular tie-back body and the tie-backbody flange, the latch member comprising upper and lower teeth adaptedto engage grooves about adjacent ends of the tubular tie-back body andthe tie-back body flange when the latch member is moved radiallyinwardly; one or more threaded members passing through and joining theriser flange and the tie-back body flange; and an insulator including aninsulation ring between the tubular tie-back body and the tie-back bodyflange and one or more insulation washers positioned between acorresponding one or more threaded members joining the riser flange withthe tie-back body flange.
 20. A connection structure for securing ariser to a subsurface wellhead assembly, comprising: an elongate,tubular connection body having first and second axial ends; a tie-backconnector for connection to the wellhead assembly, the connectorincluding an elongate tubular tie-back body and a connector assemblyradially outward of the tubular body; a wellhead engagement structure atthe first axial of the body and riser connection structure at the secondaxial end of the tubular body; a tie-back body flange mechanicallyconnected to the tie-back body, the tie-back body flange having a matingface facing upwardly from the connector assembly for connecting to theriser, said tie-back body and said riser each formed from a metallicmaterial, said riser being removable from a tie-back body flange at anupper end of said tie-back body, and said body flange being removablefrom said tie-back body for insertion or removal of said connectorassembly on said tie-back body; and an electrical insulating materialcarried at the second axial end of the tubular connection body forelectrically insulating the tubular connection body from the riser. 21.A connection structure as defined in claim 20 wherein said risercomprises a flange at an axial end of the riser.
 22. A connectionstructure as defined in claim 21 wherein said electrical insulatingmaterial is carried on the body flange.
 23. A connection structure asdefined in claim 21 wherein said body flange is threadably engaged tosaid tubular connection body.
 24. A connection structure as defined inclaim 21 wherein said body flange is connected to said tubularconnection body by a radially movable clamping element.